(a) Field of the invention:
The present invention relates to a pattern projecting system for automatic focusing endoscopes.
(B) Description of the prior art:
When using an endoscope, the operator has to perform various operations such as bending operation for changing the direction of distal end upward, downward, rightward and leftward, operation for injecting water, air, etc. into a body cavity and for sucking them out of the body cavity, operation for cutting off or taking out a sample by means of forceps etc., photographing operation, focusing operation of photographing lens, and so forth. As the operator has to perform many kinds of operations as described in the above, he has to carry out very complicated operations. Therefore, for the operator, it is desirable to omit even one of those operations and omission of focusing operation is also desirable for simplifying the operations. To omit the focusing operation, however, it is necessary to arrange the objective as a fixed focus lens system and to make its depth of focus deep by stopping down the aperture stop. When, however, the aperture stop is stopped down as above, the aperture ratio of the photographing optical system becomes small and it is necessary to make the exposure time long when photographing. However, a long exposure time when photographing an object in a body cavity becomes a cause of blur as the body cavity will move and it will become impossible to obtain a clear photograph.
To solve the above problem so that a clear photograph is obtained by a photographing optical system of large aperture ratio and to simplify the operation of the endoscope, it is necessary to automatically focus the objective. However, automatic focusing in endoscopes has the following problems. The first problem is that the object to be observed or photographed by an endoscope exists in a body cavity and, therefore, the contrast of the object is very low and the difference between the brightness and darkness of the object is very small. So, it is impossible to attain high-accuracy focus detection by a detector when the focus is detected only by the brightness of the object. The second problem is that the diameter of an endoscope is limited and cannot be made large. Therefore, it is impossible to additionally arrange an optical system for focus detection in the distal end of endoscopes. The third problem is as follows. That is, it may be considered to illuminate the object obliquely by utilizing a part of illuminating light and to use it for focus detection. In that case, however, the illuminating light for focus detection does not coincide with the optical axis of the objective, because of change in the distance to the object, when it is attempted to focus the objective by moving it. Therefore, it is not always possible to focus on the center of the field.